There are a number of pieces in the classical music repertoire that were specifically written to help learners develop their tremolo technique.
Note: Be absolutely sure to use the correct right-hand fingers in learning tremolo.
Carcassi's 25 Etudes, which are in the public domain and can be found in PDF version online, has several pieces which, if sufficiently developed, can be played in a performance. Off of the top of my head, Etude 7 and Etude 13 both emphasize tremolo.
As for the right-hand, the fingers chosen will depend on the player and the number of notes in the tremolo. A three-note tremolo might consist of a-m-i, i-m-a or even m-a-i. A four note tremolo would probably be a-m-i-a. In my experience, as long as the method of executing the technique is mechanically sound, there may be several equally viable alternatives and the best approach is to choose one and master it thoroughly.